[llvm] [TailDup][MachineSSAUpdater] Let RewriteUse insert a COPY when needed (PR #95553)

[Matt Arsenault via llvm-commits]

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@@ -236,6 +236,20 @@ void MachineSSAUpdater::RewriteUse(MachineOperand &U) {
     NewVR = GetValueInMiddleOfBlock(UseMI->getParent());
   }
 
+  // Insert a COPY if needed to satisfy register class constraints for the using
+  // MO. Or, if possible, just constrain the class for NewVR to avoid the need
+  // for a COPY.
+  const TargetRegisterClass *UseRC =
+      dyn_cast_or_null<const TargetRegisterClass *>(RegAttrs.RCOrRB);
+  if (NewVR && UseRC && !MRI->constrainRegClass(NewVR, UseRC)) {
+    MachineBasicBlock *UseBB = UseMI->getParent();
+    MachineInstr *InsertedCopy =
+        InsertNewDef(TargetOpcode::COPY, UseBB, UseBB->getFirstNonPHI(),
+                     RegAttrs, MRI, TII)
+        .addReg(NewVR);
+    NewVR = InsertedCopy->getOperand(0).getReg();
+    LLVM_DEBUG(dbgs() << "  Inserted COPY: " << *InsertedCopy << "\n");
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arsenm wrote:

Don't need the newline, it's implied in the << for MachineInstr 

https://github.com/llvm/llvm-project/pull/95553